Why do hot air balloons stay up and don't fall?

Answer 1

The difference between heat and cold is that heat is about movement and cold is about stillness. The absolute zero temperature that it is possible to reach (i.e., the coldest) is the temperature at which absolutely nothing is moving in any way. It would not be possible to reach this temperature - or anywhere close to it - on Earth by natural means.

The more molecules move (i.e., the hotter they become), the more space they need for their movements. So they expand outwards. Hot air takes up far more space than the same mass of cold air. Hence a certain volume of hot air would contain far fewer molecules than the same volume of cold air.

In other words, hot air is lighter than cold air.

By the law of gravity, what is heavy (cold) drops to the bottom, and what is light (hot) floats to the top.

The hot air in a balloon is much lighter than the cold air around it, so light that even the weight of the balloon-case and basket don't make very much difference. So your hot air floats upwards, taking the balloon and basket with it, while the cold air all around drops downwards. The balloon cannot fall down until its air is cold.

Here is a test you can try in the kitchen sink at home.

Pour a few drops of food dye into an enpty milk bottle. Then fill the bottle with boiling water, which should become dyed with your colour.

Fill a second bottle with cold water. Do not colour this second batch of water.

Now you need to turn the cold bottle upside down on top of the hot bottle. You can do this by placing a sheet of cardboard over the top of the cold bottle, then lifting it up and turning it over. When it's perfectly placed over the top of the first bottle, gently pull away the cardboard. (Honestly, a careful person can do this without spilling the water!)

Now watch what happens. You should see the coloured water moving upwards while the cold water moves downwards. The two sets of water will keep exchanging places until they have both reached the same temperature. Then both will stay still, because neither is heavier than the other.